The present invention relates to the field of turbomachines. It is aimed at the ventilation of the low-pressure turbine blades in a twin-spool gas turbine engine.
In turbomachines it is common practice to use air bled from the high-pressure, HP, compressor to cool components located in a hotter environment. These may include the HP turbine blade, bores, disks, etc.
The low-pressure, LP, turbine is one of the ventilated regions: in particular, it is contrived for air to cool the blade attachments by flowing between the blade root, its attachment and the rim of disk.
FIG. 1 depicts the turbine section of a twin-spool turbine engine. This section comprises an HP turbine stage 2 and a set of LP turbines downstream of the nozzle 4 situated between the stage 2 and the first stage of the LP turbine. The entire LP turbine here is made up of four disks bolted together to form a module. Each disk comprises a shell ring on either side of its plane. The shell rings of two adjacent disks are bolted together. Flow straighteners 5 are inserted between the various stages.
FIG. 2 depicts how the blades are attached to the LP turbine disks 3. Cavities 31 are machined at the periphery on a rim of the disks and the blades 6 are slid into these cavities and axially immobilized by an axial retaining segment 8. The segments are in the shape of arcs of a circle and are positioned bearing against one face of the rim of the disk between a hook 61 and that face 62 of the blade roots to which the hook is attached. They restrain the blades against any axial movement. The segments are scalloped and are slid into a peripheral groove 32. As can be seen, the segment is first of all angularly offset to allow the root of the blades to be inserted into its cavity then the segment is moved angularly so that the tops of the scalloped part fit in between the face of the root and the hook of each blade. As the segment is held in the groove, the assembly is axially immobilized.
Furthermore, the flow of ventilation air depicted in FIGS. 3 and 4, which illustrate two different designs of the prior art, comprises an air stream illustrated by the arrow F emanating from the nozzle DBP1 upstream of the first LP turbine stage which, for each stage, is guided between the shell ring V1 of the disk and the sealing shell ring VE, flows around the axial retaining segments 8, and reaches the turbine blade attachments.
With a view to reducing mass and to simplifying the design of the machine, the disks tend to be grouped together in pairs or in greater numbers in order to produce one-piece drums. The elements are welded together and form a unit. As can be seen in FIG. 5, a drum is made up of two disks 11 and 12 connected by a shell ring 13 on which the sealing elements 13E are created. A shell ring 14 is secured to the downstream disk 12 and comprises orifices 14A through which means of attachment, bolts not depicted in the figure, to an adjacent other group or disk can pass. In the case of a structure such as this, shell rings for the sealing elements are not needed because these are incorporated into the drum. The disks moreover have the same structure as in the earlier embodiments and the blades of the second stage of the group of this figure are also mounted in the same way. What that means in the case of the disk 12 is that the blades 6 are housed in cavities formed in the rim 12J and are axially retained by retaining segments 8 slipped both into a radial groove 12R perpendicular to the axis of the rotor 12 and between the rear face 62 of the blade root and the associated hook 61 thereof.
With a solution of this type, the issue of conveying ventilating air as far as the blade attachments arises. Air is bled from inside the drum and has to get as far as the second disk 12 of the drum. The problem does not arise in respect of the first disk. A solution whereby the rim 12J of the disk 12 is pierced at the cavity so that air can reach the attachments, as indicated by P, cannot be effected because of the stress concentrations that the drillings would cause.